Production and Excision of Thymine Damage in the DNA of Mammalian Cells Exposed to High-LET Radiations

HeLa S3 and Chinese hamster ovary cells were irradiated with high doses of carbon ions having linear energy transfers (LETs) of 170 and 780 keV/μm. The DNA was analyzed for 5,6-dihydroxydihydrothymine (t′-type) radiation products both before and after postirradiation incubation at 37°C. In HeLa cells, $2.1\times 10^{-5}$ ring-damaged thymines were produced per kilorad per 106 daltons after irradiation with high-LET carbon ions-approximately one-fifth the efficiency of t′ formation in HeLa cells exposed to low-LET X rays. t′ products were also formed less efficiently in Chinese hamster ovary cells exposed to carbon ions than in those exposed to X rays. In both cell lines, up to 80% of the t′ formed initially was excised selectively from the DNA during 60 min of postirradiation incubation at 37°C. Product excision was accompanied by small amounts of DNA degradation (less than 1%). Radiation with LET of 170 keV/μm-nearly the most effective LET for cell killing and the generation of unrejoined DNA strand breaks-produced ring-damaged thymines that were removed selectively from the DNA. This result is consistent with the conclusion that t′-type products do not contribute substantially to lethality after high-LET irradiation, although the alternative possibilities remain that t′ is not excised as efficiently after biological doses, or that a particular subclass of t′ or defective postexcision events contribute to cell killing.